The present invention relates to a solid electrolytic capacitor comprising a porous valve-acting metal substrate having formed thereon an electrically conducting polymer as a solid electrolyte layer and also relates to a method for producing the solid electrolytic capacitor.
More specifically, it relates to a high-performance solid electrolytic capacitor obtained by forming the solid electrolyte layer such that the thickness at the cut surface of the substrate and at the masking boundary part is larger than the thickness in other parts, and also relates to a method for producing the solid electrolytic capacitor, especially using a monomer solution and an oxidizing agent solution.
In the production of a solid electrolytic capacitor, as shown in FIG. 1, an oxide dielectric film layer 2 is generally formed on an anode substrate 1 comprising a metal foil subjected to an etching treatment to have a large specific surface area, a solid semiconductor layer (hereinafter referred to as a xe2x80x9csolid electrolytexe2x80x9d) 4 is formed as a counter electrode in the outer side of the dielectric layer and further thereon, an electrically conducting layer 5 such as electrically conducting paste is preferably formed, thereby fabricating a capacitor basic device. This device by itself or a stacked body resultant from stacking these devices is connected with lead wires 6,7 and thereafter, the whole is completely molded with epoxy resin 8 or the like and then put into use as a capacitor part in electric articles over a wide range.
In recent years, with the progress of digitization of electrical instruments or high-speed processing of personal computers, a compact and large-capacitance capacitor or a capacitor showing low impedance in the high frequency region is being demanded.
As the compact and large-capacitance capacitor, electrolytic capacitors such as aluminum electrolytic capacitor and tantalum electrolytic capacitor are known.
The aluminum electrolytic capacitor is advantageous in that a large-capacitance capacitor can be obtained at a low cost but suffers from such problems that when an ion conducting liquid electrolyte is used as the electrolyte, the impedance in the high frequency region is high, the capacitance decreases accompanying the evaporation of the electrolytic solution with the passing of time and the temperature characteristics are inferior.
The tantalum electrolytic capacitor where a manganese oxide is generally used as the electrolyte, has such problems that the manganese oxide predominantly produced by the thermal decomposition of manganese nitrate cannot be evaded from the possibility of the dielectric film having damages at the thermal decomposition and due to the relatively high specific resistance, the impedance in the high frequency region is high.
In order to solve these problems, it has been proposed to use an electrically conducting polymer having electric conductivity as the solid electrolyte. For example, use of an intrinsic conducting polymer having an electric conductivity of 10xe2x88x923 to 103 S/cm (see, JP-A-1-169914 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) (corresponding to U.S. Pat. No. 4,803,596)) and use of a polymer such as polyaniline (see, JP-A-61-239617), polypyrrole (see, JP-A-61-240625), a polythiophene derivative (see, JP-A-2-15611 (corresponding to U.S. Pat. No. 4,910,645)) or polyisothianaphthene (see, JP-A-62-118511) are known. These electrically conducting polymers comprising a xcfx80-conjugated structure are mostly used in the form of a composition containing a dopant.
In recent years, not only the addition of a dopant but also a combination use with, for example, manganese dioxide (see, JP-B-6-101418 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) (corresponding to U.S. Pat. No. 4,959,753)) or filler (see, JP-A-9-320901) is employed.
With respect to the shape of the solid electrolyte, it has been proposed to weld a metal onto an aluminum foil and thereby form a starting point for growing an electrically conducting polymer by the electrolytic oxidative polymerization throughout the surface of the aluminum foil (see, JP-A-4-307917).
Also, a method of performing the alternate impregnation with a monomer solution and with an oxidizing agent solution each from 1 to 20 times and the dipping in an oxidizing solution for 5 minutes to 5 hours, thereby improving the polymerization efficiency, has been proposed (see, JP-A-11-238648).
These conventional methods for producing a solid electrolytic capacitor using an electrically conducting polymer as the solid electrolyte have the following problems.
(1) When monomer is used in forming a solid electrolyte layer, a mixed solution of monomer and an oxidizing agent is used and the monomer in the mixed solution is polymerized by itself due to the oxidizing action of the oxidizing agent in the mixed solution and converts into polymer. This polymer must be discarded. Therefore, monomer in the mixed solution cannot be used effectively and the use efficiency of the starting material is very bad.
(2) The mixed solution of monomer and an oxidizing agent changes in the property and therefore, the process of forming the solid electrolyte layer is unstable, for example, the oxidizing action of the oxidizing agent decreases to shorten the life of the mixed solution.
(3) In addition, even when the oxidizing agent solution and the monomer solution are separately prepared (two solutions) and the metal foil substrate is alternately dipped in these solutions, the monomer disadvantageously dissolves out into the oxidizing agent solution at the time of dipping the substrate impregnated with the monomer solution in the oxidizing agent solution for a predetermined time period, and the monomer polymerizes in the oxidizing agent solution. As a result, the life of the oxidizing agent solution is extremely shortened. Also, in the case of dipping the metal foil substrate in the oxidizing agent solution and then in the monomer solution for a predetermined time period, the life of the monomer solution is similarly shortened.
(4) In the preparation of the mixed solution of monomer and an oxidizing agent, the concentrations of and the mixing ratio between monomer and the oxidizing agent have a certain limit, therefore, the monomer concentration cannot be freely selected and for forming the solid electrolyte layer to have a desired thickness, for example, the number of times of performing the polymerization must be disadvantageously increased.
(5) According to the method of repeating the alternate dipping in the oxidizing agent solution and in the monomer solution to perform the polymerization, a washing step is generally provided after each polymerization. This washing operation every each polymerization and the time spent therefor not only lower the production efficiency of a solid electrolytic capacitor device but also decrease the strength of the polymer solid electrolyte layer part, because overlapping of the polymer solid electrolyte layers is reduced. Thus, improvement is necessary also in view of the capability of the solid electrolytic capacitor.
(6) Since the dielectric layer is formed only by an electrochemical forming treatment in the later step, the cut surface (cut end part) formed in cutting the porous valve-acting metal into a predetermined shape is weak as compared with the part other than the cut surface, and the solid electrolyte is liable to adhere thereto in a small amount.
(7) In the masking part for insulating the anode part from the cathode part of the solid electrolytic capacitor and thereby preventing the solid electrolyte from extending to the anode part, the solid electrolyte is liable to fail in satisfactorily adhering and the capacitance is disadvantageously reduced. Furthermore, the masking part works out to a part of joining anodes in stacking the solid electrolytic capacitor devices and therefore, this part is readily subject to a stress, as a result, a large leakage current is generated and short circuit readily takes place.
(8) In the method of forming the solid electrolyte of an electrically conducting polymer through the dipping in a solution containing a monomer of organic polymer and in a solution containing an oxidizing agent, as described above, the amount of the solid electrolyte adhered is liable to be large in the center part of the substrate (aluminum foil) and small at the cut end part on the cut surface and at the masking part. Furthermore, the balance in the adhering state between the oxidizing agent and the monomer is readily lost to fail in obtaining a polymer having constant performance.
As a result of extensive investigations for forming a solid electrolyte at the cut end part and in the masking boundary region while taking account of the above-described problems, the present inventors have found that the solid electrolyte can be formed in an increased amount particularly on the cut surface and in the masking boundary region by gradually vaporizing the solvent of an oxidizing agent solution on the substrate and polymerizing a monomer on the remaining oxidizing agent. The solid electrolytic capacitor obtained as such is verified to show improved adhesion between the dielectric layer and the solid electrolyte formed on the dielectric film and to have excellent stability in various basic properties such as capacitance, dielectric loss (tan xcex4), leakage current and short circuit defective ratio, and also in the reflow soldering heat resistance and the moisture resistance load characteristics.
Furthermore, in the method where the solid electrolytic capacitor having the excellent properties as described above is produced by dipping the substrate into monomer solution of organic polymer or dispersant (hereinafter it is referred to simply as a solution containing a monomer) and solution of an oxidizing agent or dispersant (hereinafter it is referred to simply as a solution containing an oxidizing agent), the present inventors have found that for reducing the elution of the monomer into the solution containing an oxidizing agent or the elution of the solution containing an oxidizing agent into the solution containing a monomer, it is effective to control the dipping time in the solution containing an oxidizing agent or in the solution containing a monomer and also to control the polymerization time by adding a drying step at a predetermined temperature for a predetermined time after the dipping in the solution containing a monomer. Furthermore, it has been found that when the washing is performed at the final stage after the alternate dipping in the solution containing a monomer and in the solution containing an oxidizing agent, the drying and the polymerization are repeated a predetermined number of times, the overlapping between layers can be maintained and the solid electrolyte formed can have excellent properties.
The present invention provides a solid electrolytic capacitor and a method for producing the solid electrolytic capacitor as described below.
1. A solid electrolytic capacitor comprising a porous valve-acting metal substrate having on the surface thereof a dielectric film and having provided on the dielectric film a solid electrolyte of an electrically conducting polymer obtainable by oxidation-polymerizing a monomer of organic polymer using an oxidizing agent, wherein the thickness of the solid electrolyte layer in the peripheral part of the substrate is larger than the thickness of the solid electrolyte layer in the center part of the substrate.
2. A solid electrolytic capacitor comprising a porous valve-acting metal substrate having on the surface thereof a dielectric film and having provided on the dielectric film a solid electrolyte of an electrically conducting polymer obtainable by oxidation-polymerizing a monomer of organic polymer using an oxidizing agent, said valve-acting metal porous substrate being cut into a predetermined shape, wherein the thickness of the solid electrolyte layer in the periphery of the cut surface of the substrate is larger than the thickness of the solid electrolyte layer in the center part of the substrate.
3. A solid electrolytic capacitor comprising a porous valve-acting metal substrate having on the surface thereof a dielectric film and having provided on the dielectric film a solid electrolyte of an electrically conducting polymer obtainable by oxidation-polymerizing a monomer of organic polymer using an oxidizing agent, said valve-acting metal porous substrate being cut into a predetermined shape, wherein the thickness of the solid electrolyte layer in the periphery of the cut surface of the substrate and in the masking boundary part is larger than the thickness of the solid electrolyte layer in the center part of the substrate.
4. The solid electrolytic capacitor as described in any one of 1 to 3 above, wherein the partiality in the thickness of said solid electrolyte is created by the solution chemical oxidative polymerization or vapor phase chemical oxidative polymerization of a monomer of organic polymer performed on said valve-acting metal substrate having thereon a dielectric film.
5. The solid electrolytic capacitor as described in 4 above, wherein the partiality in the thickness of said solid electrolyte is created by repeating an operation of alternately dipping said valve-acting metal substrate having thereon a dielectric film in a solution containing a monomer of organic polymer and in a solution containing an oxidizing agent.
6. The solid electrolytic capacitor as described in any one of 1 to 5 above, wherein the porous valve-acting metal substrate has a plate- or foil-like shape.
7. The solid electrolytic capacitor as described in 6 above, wherein the solid electrolyte is formed such that the center part of the porous valve-acting metal substrate has a guitar- or gourd-like cross section in the longitudinal direction and in the transverse direction.
8. The solid electrolytic capacitor as described in 7 above, wherein in the cross section of the center part of the substrate on which a solid electrolyte layer is formed, the difference between the maximum thickness and the minimum thickness is from 0 to 200 xcexcm.
9. The solid electrolytic capacitor as described in any one of 1 to 8 above, wherein the porous valve-acting metal is a simple metal selected from aluminum, tantalum, niobium and titanium, or an alloy thereof.
10. The solid electrolytic capacitor as described in any one of 1 to 9 above, wherein the monomer of organic polymer for forming the electrically conducting polymer is a compound containing a 5-member heterocyclic ring, or a compound having an aniline skeleton.
11. The solid electrolytic capacitor as described in 10 above, wherein the compound containing a 5-member heterocyclic ring is a compound having a thiophene skeleton or a polycyclic sulfide skeleton.
12. The solid electrolytic capacitor as described in 11 above, wherein the monomer compound having a thiophene skeleton is 3-ethylthiophene, 3-hexylthiophene, 3,4-dimethylthiophene, 3,4-methylenedioxythiophene, and 3,4-ethylenedioxythiophene.
13. A solid electrolytic multilayer capacitor obtainable by stacking a plurality of sheets of the solid electrolytic capacitor as described in 1 to 12 above.
14. A method for producing a solid electrolytic capacitor, comprising providing a solid electrolyte of an electrically conducting polymer on a dielectric film on the surface of a porous valve-acting metal substrate by the oxidative polymerization of a monomer of organic polymer using an oxidizing agent, wherein the solid electrolyte layer is formed such that the thickness in the peripheral part of the substrate is larger than the thickness in the center part of the substrate.
15. The method for producing a solid electrolytic capacitor as described in 14 above, wherein the partiality in the thickness of said solid electrolyte is created by the solution chemical oxidative polymerization or vapor phase chemical oxidative polymerization of a monomer of organic polymer performed on said valve-acting metal substrate having thereon a dielectric film.
16. The method for producing a solid electrolytic capacitor as described in 15 above, wherein the partiality of the thickness of said solid electrolyte is created by repeating an operation of alternately dipping said valve-acting metal substrate having thereon a dielectric film in a solution containing a monomer of organic polymer and in a solution containing an oxidizing agent.
17. The method for producing a solid electrolytic capacitor as described in 16 above, wherein the substrate is alternately dipped in a each solution for less than 5 minutes.
18. The method for producing a solid electrolytic capacitor as described in 16 above, wherein the alternate dipping is repeated from 15 to 30 times.
19. The method for producing a solid electrolytic capacitor as described in 16 above, wherein said valve acting metal substrate is left standing in air for from 5 seconds to 15 minutes after the dipping in the solution containing a monomer.
20. The method for producing a solid electrolytic capacitor as described in 16 above, wherein said valve acting metal substrate is left standing in air for from 10 seconds to 15 minutes after the dipping in said solution containing an oxidizing agent.
21. The method for producing a solid electrolytic capacitor as described in 19 or 20 above, wherein said valve acting metal substrate is left standing in air at a temperature of 0 to 60xc2x0 C.
22. The method for producing a solid electrolytic capacitor as described in 16 above, wherein after the step of dipping said valve acting metal substrate alternately in the solution containing a monomer and in the solution containing an oxidizing agent to perform the polymerization, said valve acting metal substrate is washed.
23. A method for producing a solid electrolytic capacitor, comprising forming a solid electrolyte layer of an electrically conducting polymer on the surface of a valve acting metal substrate having thereon a dielectric film porous body, using solution containing a monomer capable of forming an electrically conducting polymer under the action of an oxidizing agent, and a solution containing an oxidizing agent, which has a step of dipping said valve acting metal substrate alternately in the solution containing a monomer and in the solution containing an oxidizing agent each for less than 5 minutes to perform the polymerization.
24. A method for producing a solid electrolytic capacitor, comprising forming a solid electrolyte layer of an electrically conducting polymer on the surface of a valve acting metal substrate having thereon a dielectric film porous body, using a solution containing a monomer capable of forming an electrically conducting polymer under the action of an oxidizing agent, and a solution containing an oxidizing agent, wherein a step of dipping said valve acting metal substrate alternately in the solution containing a monomer and in the solution containing an oxidizing agent each for less than 5 minutes is repeated from 15 to 30 times to perform the polymerization.
25. The method for producing a solid electrolytic capacitor as described in 23 or 24 above, wherein said valve acting metal substrate is left standing in air for from 5 seconds to 15 minutes after the dipping in the solution containing a monomer.
26. The method for producing a solid electrolytic capacitor as described in any one of 23 to 25 above, wherein said valve acting metal substrate is left standing in air for from 10 seconds to 15 minutes after the dipping in said solution containing an oxidizing agent.
27. The method for producing a solid electrolytic capacitor as described in 25 or 26 above, wherein said valve acting metal substrate is left standing in air at a temperature of 0 to 60xc2x0 C.
28. The method for producing a solid electrolytic capacitor as described in any one of 23 to 27 above, wherein after repeating the step of performing polymerization, said valve acting metal substrate is washed.
29. The method for producing a solid electrolytic capacitor as described in any one of 23 to 28 above, wherein a part of the solid electrolyte layer formed of an electrically conducting polymer has a lamellar structure or a fibril structure.
30. The method for producing a solid electrolytic capacitor as described in any one of 23 to 29 above, wherein the monomer for forming an electrically conducting polymer is a compound containing a heterocyclic 5-membered ring.
31. The method for producing a solid electrolytic capacitor as described in any one of 23 to 30 above, wherein the monomer for forming an electrically conducting polymer is a compound having an aniline skeleton.
32. The method for producing a solid electrolytic capacitor as described in 30 above, wherein the compound containing a heterocyclic 5-membered ring has a thiophene skeleton.
33. The method for producing a solid electrolytic capacitor as described in 30 above, wherein the monomer for forming an electrically conducting polymer is selected from the group consisting of 3-ethylthiophene, 3-hexylthiophene, 3,4-dimethylthiophene, 3,4-methylenedioxythiophene, 3,4-ethylenedioxythiophene, 1,3-dihydroisothianaphthene, and 3,4-ethylenedioxyfurane.
34. The method for producing a solid electrolytic capacitor as described in any of 23 to 33 above, wherein said valve acting metal is a single metal selected from aluminum, tantalum, niobium and titanium, or an alloy thereof.
35. A solid electrolytic capacitor obtained by the production method described in 23 above.
36. A solid electrolytic capacitor obtained by the production method described in 24 above.
37. A solid electrolytic capacitor obtained by the production method described in any one of 25 to 34 above.